notif: Add setup for notification channel migrations

Author: rajveermalviya

lib/notifications/display.dart

@@ -51,11 +51,27 @@ class NotificationChannelManager {
   //    settings for the channel -- like "override Do Not Disturb", or "use
   //    a different sound", or "don't pop on screen" -- their changes get
   //    reset.  So this has to be done sparingly.
-  //
-  //    If we do this, we should also look for any channel with the old
-  //    channel ID and delete it.  See zulip-mobile's `createNotificationChannel`
-  //    in android/app/src/main/java/com/zulipmobile/notifications/NotificationChannelManager.kt .
   static Future<void> _ensureChannel() async {
+    // See if our current-version channel already exists; delete any obsolete
+    // previous channels.
+    var found = false;
+    final channels = await _androidHost.getNotificationChannels();
+    for (final channel in channels) {
+      assert(channel != null); // TODO(flutter#97848)
+      if (channel!.id == kChannelId) {
+        found = true;
+      } else {
+        await _androidHost.deleteNotificationChannel(channel.id);
+      }
+    }
+
+    if (found) {
+      // The channel already exists; nothing to do.
+      return;
+    }
+
+    // The channel doesn't exist.  Create it.
+
     await _androidHost.createNotificationChannel(NotificationChannel(
       id: kChannelId,
       name: 'Messages', // TODO(i18n)

test/model/binding.dart

@@ -540,15 +540,13 @@ class FakeAndroidNotificationHostApi implements AndroidNotificationHostApi {
   List<NotificationChannel> _createdChannels = [];
 
   @override
-  Future<List<NotificationChannel?>> getNotificationChannels() {
-    // TODO: implement getNotificationChannels
-    throw UnimplementedError();
+  Future<List<NotificationChannel?>> getNotificationChannels() async {
+    return _createdChannels.toList(growable: false);
   }
 
   @override
-  Future<void> deleteNotificationChannel(String channelId) {
-    // TODO: implement deleteNotificationChannel
-    throw UnimplementedError();
+  Future<void> deleteNotificationChannel(String channelId) async {
+    _createdChannels.removeWhere((e) => e.id == channelId);
   }
 
   @override